Dark Matter Stars Could Solve Cosmic Mystery

This artist's conception shows what an invisible "dark star" might look like when viewed in infrared light that it emits as heat. The core is enveloped by clouds of hydrogen and helium gas. A new University of Utah study suggests the first stars in the universe did not shine, but may have been dark stars.

Credit: University of Utah

In the early universe, the first stars may have been made
not of regular matter, but of its mysterious cousin, dark matter. But exactly
how it all happened remains a mystery, and figuring it out could help
astronomers understand dark matter itself.

Dark matter is the pesky substance thought to permeate the
universe that stubbornly refuses to show itself to telescopes or any other
direct detection method scientists can throw at it. Yet researchers can sense
it lurking by the gravitational pull dark matter exerts on normal stars and
galaxies.

There are many competing theories about what dark matter is
actually made of, and each suggestion comes with slightly different properties
and implications. Now, a new study calculated which possible forms of dark matter
could lead to so-called dark
stars.

Not really dark

"Not all kinds of dark matter would be able to form
dark stars," said study leader Paolo Gondolo of the University of Utah. "In
this sense dark stars are a tool to understand the nature of dark matter."
[Video:
Dark Matter in 3-D]

The term dark star is somewhat misleading, he said, because
in fact these stars would emit light and would be visible. But the matter that
reacts in the star's core to form that light would be dark
matter, not regular matter.

Though dark stars have not yet been observed by telescopes,
some experts think future observatories like the James Webb Space Telescope, a
successor to the Hubble Space Telescope, set to launch in 2014, could be up to
the job.

"If we detect evidence of dark stars, or if we can say
that there are no dark stars, then they present constraints" on what dark
matter is made of, Gondolo told SPACE.com.

What makes a star?

Many dark matter candidate particles are their own
antimatter partner, which means if two particles come close enough together,
they will annihilate to produce energy, some of which is emitted in the form of
light.

Today, with the universe spread out, matter is not packed
tightly enough for dark matter particles to be condensed to the point of
annihilating. But after the Big Bang when the universe was young and
comparatively small, the conditions were right for dark stars.

Dark matter would only form a very small fraction of the
total mass of such stars ? the rest would be normal matter. But the dark matter
annihilation process is very efficient, because colliding dark particles would
convert all of their mass to energy via Einstein's equation, E=mc2.

Thus dark stars would shine quite bright.

Eventually, when the dark matter supply inside the star was
exhausted, normal matter would condense to begin the regular process of nuclear
fusion that fuels most stars. Some of these could still be around today.

A few good candidates

One popular candidate for the stuff that makes up dark
matter is an elementary particle called the axion. These hypothetical particles
are predicted by theory, but have never been detected.

They would have no electric charge, a small mass, and would
be very resistant to interact with ordinary matter ? a key qualification that
makes some scientists suspect they are the culprit behind dark matter.

"We find that axions are not good to form dark
stars," Gondolo said. Their particular characteristics are not suitable
for reacting in the way it would take to power stars. That means that if dark
stars are ever observed, axions may be out as a candidate.

Another possibility is a hypothetical category of particles
called WIMPs
(weakly interacting massive particles). These would be quite heavy ? which
could help explain why dark matter accounts for about 98 percent of all the
matter in the universe, scientists think.

The weakly interacting part denotes their standoffish nature
? they would rarely react with other particles ? again, another requirement if
they are the stuff
of dark matter.

Some WIMPS, it turns out, would be good at forming dark
stars. One type, a particle called the neutralino, is predicted by the
supersymmetric theory, which posits that every particle has a symmetric partner
particle that we haven't yet discovered.

One kind of neutralino, the photino (the partner of the
photon) would be the lightest ? and therefore the easiest to detect ? of these
partners.

"Neutralinos can produce dark stars, with very few
exceptions," Gondolo said.

The findings were published in a recent issue of the Journal
of Cosmology and Astroparticle Physics.

Clara has been SPACE.com's Assistant Managing Editor since 2011, and has been writing for SPACE.com and LiveScience since 2008. Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what her latest project is, you can follow Clara on Google+.